In the case of forming an image on a large-scale screen of a movie theater, a planetarium, etc., when attempting to project a single image over the entire screen by means of a single projector, extremely high performance is required for such projector, leading to high cost. Thus, a multi-projection device which projects, onto a screen, images by means of a plurality of projectors and combines the plurality of projected images at their ends so as to form a single large image has been proposed. In such multi-projection device, in order to prevent a border line from appearing in the part where a plurality of images are combined, having the ends of images overlap one another has generally been employed.
However, when combining projected images with their ends being made to overlap one another, this will invite a new problem in that the combined part has a higher image brightness than that of the surroundings, which gives a viewer a feeling of discomfort. Therefore, a measure to arrange a light shielding plate in front of a projection lens of each of the projectors so as to cause the brightness of such combined part to be equal to that of the surroundings has been employed. Such light shielding plate attenuates the brightness of the end of an image projected from each of the projectors, thereby adjusting the image brightness of the combined part so as to be substantially equal to that of the surroundings.
The arrangement of such light shielding plate can prevent the combined part of images from having an extremely high brightness. However, it is difficult for the light shielding plate to completely control the traveling of light projected from a projector, and this has led to a further new problem. That is, the light projected from a projector is diffracted by the light shielding plate, which does not allow for smooth attenuation of the brightness at the end of an image, resulting in the appearance of a striped, high brightness part around the combined part.
The light shielding plate disclosed in Japanese Patent No. 3,737,029 is known to solve the above problem. Such light shielding plate is formed by laminating a large number of thin-plate filters. Each of the filters has an end formed into a sawtooth shape comprised of isosceles triangles placed in a line.
Further, such light shielding plate is configured such that white-light transmittance decreases in a stepwise manner in the direction away from the sawtooth-shaped ends. In further detail, such many filters are arranged with their ends being shifted from one another, so that the filters are laminated such that the light shielding plate has an increasing thickness in the direction away from the sawtooth-shaped ends. Therefore, the light shielding plate is configured to have a lower white-light transmittance at a location thereof where a larger number of filters are laminated.
The employment of the above-described light shielding plate which has sawtooth-shaped ends and which has a decreasing white-light transmittance in the direction away from such ends achieves smooth attenuation of the brightness at the end of an image, and this can reduce the striped, high brightness part which would otherwise appear in the combined part.